Homeless people would drop like flies if they didn't have fairly good immune systems from being outside a lot and from being exposed to germs and viruses due to their surroundings.

EDIT - how is this incorrect? Looking at some of their surroundings, homeless people must have fairly good immune systems as a group. It's because they are outside (Vitamin D and C) and are active. Most aren't obese. Most aren't older than 60s. Guess that bot got me...